Allow recipes to set previous operation

[gustaphe]

Previously, recipes had no way of affecting the previous operation. With this (somewhat ugly) change, a type that fundamentally represents for instance a multiplication or sum can tell latexify, so that things like parentheses work out right.

Part of fixing #280

TODO

• Documentation
• Maybe more tests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.81%. Comparing base (6af2a54) to head (1577e93).
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #292      +/-   ##
==========================================
- Coverage   81.78%   77.81%   -3.97%     
==========================================
  Files          21       21              
  Lines         818      834      +16     
==========================================
- Hits          669      649      -20     
- Misses        149      185      +36     

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[gustaphe]

Without this,

julia> struct SimpleLength
x
end

julia> @latexrecipe function f(x::SimpleLength)
return Expr(:latexifymerge, x.x, "\\;\\mathrm{m}")
end

julia> @latexify $(SimpleLength(3))^2

$$3\;\mathrm{m}^2$$

With this

julia> @latexrecipe function f(x::SimpleLength)
operation := :*
return Expr(:latexifymerge, x.x, "\\;\\mathrm{m}")
end

julia> @latexify $(SimpleLength(3))^2

$$\left( 3\;\mathrm{m} \right)^{2}$$